NP205 Speedo leak

[Accelo]

Go ahead and us sealant. I use Permatex 82180 Ultra Black Maximum Oil Resistance for this application. However, I would use what I had. Pipe sealant would also work.
I don't believe GM used a gasket, and the surface would be very small if they did.
The speedometers on the 205's were built before GM put a serious effort into oil leaks.
The early sleeve nuts didn't even had a seal installed in them. Look carefully for wear on the gear seal surface and for excessive play of the gear in the nut bore.